Tar sands (also known as oil or bituminous sands) are an extensive source of hydrocarbons which in recent years have become economically viable. There are two commercial plants presently operating in the Alberta tar sands; the first produces approximately 50,000 barrels of synthetic crude/day, the second about 130,000 barrels/day. This second plant, owned by the assignees of this invention, is hereafter referred to as `the aforementioned plant`.
Both these plants use an extraction process known as the `hot water process`. This process is well described in the literature (for example, see Canadian Pat. No. 1,055,868).
The hot water process produces a waste stream, called "tailings", which is a mixture of solids, water and some bitumen. The solids comprise a coarse sand fraction and a fines fraction usually described as being -44.mu. clay and silt particles.
The volume of this waste stream is very large. In the aforementioned plant, dry tar sand feed enters the process at a rate of about 13,000 tons/hour and tailings leave at a rate of about 20,000 tons/hour.
The plant design analysis of the aforementioned plant specifies that the bitumen content of the tailings should be about 0.49% by weight. However the actual content varies widely depending on whether the tar sand feed is high or low grade. Bitumen losses by way of the tailings increase as the fines content of the tar sand feed increases. Bitumen losses with the tailings at the aforementioned plant have been known to run as high as 11,000 barrels/day when it was processing high fines feed.
The tailings are discarded from the extraction plant into a tailings pond. In the case of the 130,000 barrel/day plant, this pond will eventually cover about 10 square miles. The bitumen, while constituting an undesirable process loss, also is a serious environmental problem--because it floats on the surface of this large body of water, it can coat wild fowl landing in it.
Clearly it would be desirable to contain the bitumen within booms and recover it using an oil skimmer. However, several types of commercially available skimmers have been tried with poor results. This failure is attributable to the peculiar nature of the bitumen.
More particularly, bitumen is a particularly viscous material. At 10.degree. C., its viscosity is in the order of 2,000,000 centistokes; at 20.degree. C. it is in the order of 200,000 centistokes. By way of comparison, crude oil and bunker oil at 20.degree. C. have viscosities in the order of 300 and 1000 centistokes respectively.
When the hot bitumen discharged by the extraction plant contacts the cold water of the tailings pond, its lower most portion forms into a cohesive stiff layer which supports an uppermost portion having a butter-like consistency. This mass is hereinafter referred to as a bitumen blanket.
When existing commercially available oil skimmers were applied to the bitumen blanket, it was found that, instead of picking the bitumen up, the rotating discs, drums and conveyor belts of these units would tend to ride up onto the blanket and pass it beneath them. The machines also tended to break the blanket up without picking up the bitumen. Finally, some of them would tend to clog up with the bitumen.
With the foregoing in mind, it was therefore the object of this invention to devise a machine which would successfully recover a bitumen blanket floating on the surface of a body of water.